Chapter 13: Conjugated Unsaturated Systems

Conjugation, Resonance, and Dienes

Conjugated unsaturated systems are fundamental in organic chemistry, involving molecules with alternating single and multiple bonds that allow for electron delocalization. This chapter introduces the concepts of conjugation, resonance, and the chemistry of dienes, with a focus on their stability and reactivity.

Definition of Conjugation

• Conjugation occurs when p orbitals are located on three or more adjacent atoms, allowing for the overlap of these orbitals and the delocalization of electrons.

• Examples include 1,3-diene and allylic carbocation, where four adjacent p orbitals form a conjugated system.

• Each carbon in a conjugated system is typically sp2 hybridized and contains a p orbital with one electron.

Stability of Conjugated Systems

• Having three or more p orbitals on adjacent atoms allows for p orbital overlap and electron delocalization.

• Delocalization of π electrons over a larger volume lowers the energy of the molecule, increasing its stability.

• The electron density in the π bonds is spread out, making the molecule more stable than isolated double bonds.

Allylic Radical Substitution vs. Traditional Addition to Double Bonds

Allylic radical substitution and electrophilic addition are two different reaction pathways for alkenes, depending on the reaction conditions.

• At low temperature, halogens add across the double bond (addition reaction).

• At high temperature or very low halogen concentration, allylic substitution occurs, forming an allylic halide.

• Example: Propene reacts with Br2 to give different products under different conditions.

Allylic Chlorination (High Temperature) - Mechanism

Allylic chlorination is a free radical chain reaction that occurs at high temperature in the gas phase.

• Initiation step: Formation of chlorine radicals.

• Propagation step 1: Formation of an allyl radical by abstraction of an allylic hydrogen (lower bond dissociation energy for allylic C–H bonds).

• Propagation step 2: Allyl radical reacts with Cl2 to form allyl chloride and regenerate a chlorine radical.

• Example reaction:

Relative Stability of Carbon Radicals

The stability of carbon-centered radicals is influenced by the degree of substitution and resonance stabilization.

• Order of stability: allyl > 3° > 2° > 1° > vinyl

• Allylic radicals are stabilized by resonance due to the overlap of adjacent p orbitals.

Allylic Bromination with N-Bromosuccinimide (NBS)

Allylic bromination is a selective method for introducing a bromine atom at the allylic position of alkenes using NBS in the presence of light or peroxides.

• NBS provides a continuous low concentration of bromine for the radical reaction.

• Low bromine concentration favors allylic substitution over addition to the double bond.

• Example reaction:

Mechanism of Allylic Bromination with NBS

• Initiation: Formation of bromine radicals by exposure of NBS to light or peroxides.

• Propagation steps:

  1. A bromine radical abstracts an allylic hydrogen from propene, forming an allylic radical and HBr.

  2. HBr reacts with NBS to produce a bromine molecule (Br2).

  3. The bromine molecule reacts with the allylic radical to form the allylic bromide and regenerate a bromine radical.

• Key equations:

Stability of Allylic Radicals

• Allylic radicals are more stable than primary, secondary, tertiary, or vinyl radicals due to resonance stabilization.

• The overlap of adjacent p orbitals allows the unpaired electron to be delocalized over multiple atoms, lowering the energy of the radical.

• Relative stability: allyl > 3° > 2° > 1° > vinyl